If you set "editor.contentFilter.allowIFrame" value to false, insertion of <iframe> tag by the user is restricted and the tag is automatically removed.
synapeditor.config.js
{
'editor.contentFilter.allowIFrame': false
}
If you set "editor.contentFilter.allowIScript" value to false, insertion of <script> tag by the user is restricted and the tag is automatically removed.
synapeditor.config.js
{
'editor.contentFilter.allowScript': false
}